1. Field of the Invention
The present invention relates to a PIN type semiconductor photoelectric conversion device using a non-single-crystal semiconductor and a method of making the same.
2. Description of the Prior Art
Heretofore there has been proposed a PIN type semiconductor photoelectric conversion device using a non-single-crystal semiconductor.
The PIN type semiconductor photoelectric conversion device comprises a non-single-crystal semiconductor laminate member having a first non-single-crystal semiconductor layer of a first conductivity type (P or N conductivity type), an I-type non-single-crystal semiconductor layer and a third non-single-crystal semiconductor layer of a second conductivity type reverse from the first conductivity type (that is, N-type when the first non-single-crystal semiconductor layer is P-type, and P-type when the latter is N-type), and first and second electrodes making ohmic contact with the first and third non-single-crystal semiconductor layers, respectively.
When irradiated by light on the side of the first non-single-crystal semiconductor layer of the non-single-crystal semiconductor laminate member, such a PIN type semiconductor photoelectric conversion device performs a photoelectric conversion through the following mechanism.
That is to say, the incident light passes through the first non-single-crystal semiconductor layer to reach the second non-single-crystal semiconductor layer, wherein it is absorbed. In consequence, carriers (electron-holes pairs) are created in the second non-single-crystal semiconductor layer. The carriers migrate into the first and second non-single-crystal semiconductor layers and then reach the first and second electrodes. Accordingly, a current corresponding to the intensity of the incident light is supplied to a load connected across the first and second electrodes.
In the abovesaid PIN type semiconductor photoelectric conversion device, the first non-single-crystal semiconductor layer serves as a window for the light incident on the second non-single-crystal semiconductor layer. Therefore, it is desired that the first non-single-crystal semiconductor layer be small in light absorption coefficient.
The second non-single-crystal semiconductor layer absorbs the incident light and generates carriers. It is therefore desirable that the second non-single-crystal semiconductor layer be large in light absorption coefficient.
Moreover, the first and third non-single-crystal semiconductor layers receive the carriers created in the second non-single-crystal semiconductor layer and directs them to the first and second electrodes, respectively. For this reason, it is desirable that the first and third non-single-crystal semiconductor layers be high in electric conductivity.
Incidentally, there has been proposed a PIN type semiconductor photoelectric conversion device in which the first, second and third non-single-crystal semiconductor layers each have an amorphous structure.
Generally, in the case of such a PIN type semiconductor photoelectric conversion device, each non-single-crystal semiconductor layer of the amorphous structure can be formed to have a large light absorption coefficient which is close to that of a semiconductor layer having a single-crystal structure, so that it is possible that light incident on the second non-single-crystal semiconductor layer through the first non-single-crystal semiconductor layer is effectively absorbed to create carriers.
In the case where the first, second and third non-single-crystal semiconductor layers are each formed by a non-single-crystal semiconductor layer of the amorphous structure having a large light absorption coefficient, however, the first non-single-crystal semiconductor layer serving as a window for the light to be entered into the second non-single-crystal semiconductor layer is equipped with a large light absorption coefficient, and hence is poor in the function as the window.
When the first, second and third non-single-crystal semiconductor layers ar each formed by a non-single-crystal semiconductor layer of the amorphous structure having a small light absorption coefficient, the effect of the window action of the first non-single-crystal semiconductor layer can be heightened but the efficiency of absorbing the incident light by the second non-single-crystal semiconductor layer to create carriers is low.
Furthermore, in the case of the PIN type semiconductor photoelectric conversion device in which the first, second and third non-single-crystal semiconductor layers all have the amorphous structure, the electric conductivity of each non-single-crystal semiconductor layer is usually as low as 1/10.sup.6 to 1/10.sup.8 that of the single-crystal structure, so that the carriers created in the second non-single-crystal semiconductor layer cannot quickly be delivered by the first and third non-single-crystal semiconductor layers to the first and second electrodes without loss.
Accordingly, the PIN type semiconductor photoelectric conversion device of this construction has the defects of low photoelectric conversion efficiency and poor light response characteristic.
Besides, there has also been proposed a PIN type semiconductor photoelectric conversion device in which the first, second and third non-single-crystal semiconductor layers each have a microcrystalline structure.
In the case of such a PIN type photoelectric conversion device, since the non-single-crystal semiconductor layer of the microcrystalline structure can generally be formed to have a electric conductivity higher than does the non-single-crystal semiconductor layer of the amorphous structure, the carriers generated in the second non-single-crystal semiconductor layer can be delivered by the first and third non-single-crystal semiconductor layers to the first and second electrodes with less loss and more quickly than in the case of the first, second and third non-single-crystal semiconductor layers of the amorphous structure.
However, the electric conductivity of the non-single-crystal semiconductor layer of the microcrystalline structure is as low as 1/10.sup.2 to 1/10.sup.4 that of the semiconductor layer of the single-crystal structure. On account of this, the carriers created in the second non-single-crystal semiconductor layer cannot rapidly be delivered by the first and third non-single-crystal semiconductor layers to the first and second electrodes without loss.
In general, the non-single-crystal semiconductor layer of the microcrystalline structure can be formed to have a light absorption coefficient smaller than that of the non-single-crystal semiconductor layer of the amorphous structure.
In this case, however, it is impossible to concurrently and sufficiently satisfy the requirements of absorbing incident light by the second non-single-crystal semiconductor layer with high efficiency to create carriers and quickly delivering the carriers by the first and third non-single-crystal semiconductor layers from the second non-single-crystal semiconductor layer to the first and second electrodes without loss.
Accordingly, the PIN type semiconductor photoelectric conversion device of the type that the first to third non-single-crystal semiconductor layers all have the microcrystalline structure is low in photoelectric conversion efficiency and in light response characteristic as well.
Furthermore, this PIN type semiconductor photoelectric conversion device is not easy to fabricate because high temperature is generally required for the formation of the non-single-crystal semiconductor layer of the microcrystalline structure.
In addition, whether the first to third non-single-crystal semiconductor layers have the amorphous or microcrystalline structure, the conventional PIN type semiconductor photoelectric conversion devices do not possess the construction that makes use of that portion of the light incident on the second non-single-crystal semiconductor layer through the first non-single-crystal semiconductor layer which is not absorbed by the second non-single-crystal semiconductor layer and directed to the second electrode.
Therefore, the prior art PIN type semiconductor photoelectric conversion devices have the drawback of low photoelectric conversion efficiency.